1. Field of the Invention
The present invention relates to a method and device for metering ink in an offset printing press.
2. Description of the Prior Art
Offset printing presses are well known in the printing art. In an offset printing press, an image or plate cylinder which contains on its outer surface an image to be printed rotates in contact with either the paper upon which the image is to be printed, or preferably, rotates in contact with a transfer or blanket cylinder which transfers the image from the image cylinder to the paper. Ink is transferred to the image cylinder, which ink is thereafter transferred either directly to the paper or to the transfer cylinder and then to the paper.
There are several known ways in which ink may be fed to an image cylinder in an offset printing press. The most common way of feeding ink to an image cylinder is the use of an ink fountain. An ink fountain is a device in which a fountain roll has mounted against it a trough which is filled with ink. At the interface between the bottom of the trough (i.e., the ink fountain) and the fountain roll are located a series of ink keys. The ink keys may be moved toward and away from the fountain roll, and the distance between the end of each ink key and the surface of the fountain roll determines the thickness of ink applied by the ink key on the outer surface of the fountain roll. The fountain roll is in close proximity with, in an area past the ink keys in the direction of rotation of the fountain roll, a metering roll.
Metering rolls used with prior art ink fountains normally contain a spiral or "barber pole" groove on the outer surface. This spiral groove assists in removing contaminants from the metering roll. The contaminants flow into the spiral groove, and are removed from the end of the metering roll by the spiral shape of the groove in conjunction with the rotation of the metering roll. Because the contaminants are contained in the spiral groove, they are not transferred to any downstream rolls, and therefore the image cylinder is not contaminated. However, the use of a spiral groove on the metering roll causes a flute pattern of differing ink thicknesses on the metering roll. The flute pattern must be smoothed out from any ink film before it is transferred to the image cylinder. In order to smooth out the flute pattern from a metering roll, prior art devices have used an ink train containing a large number of smoothing rollers (commonly eleven to fifteen) between the metering roll and the image cylinder. The ink is transferred, sequentially, from the metering roll to each of the smoothing rollers before it is transferred to the image cylinder. Each sequential step of ink transfer from one smoothing roller to another incrementally smooths out the flute pattern in the ink from the metering roll. As will be readily understood, the need for a large number of smoothing rollers complicates the printing press apparatus and increases costs.
Another problem with the use of ink fountains is weeping. Weeping is caused by drops which accumulate at the ink key/fountain roll interface. These drops can drip onto portions of the printing press, including the ink train, thereby altering the printed image or the condition of the ink fed to the image roll. Additionally, heat can build up as the result of contact between the rigid ink keys and the fountain roll. This heat build-up can alter conditions of the ink or the rollers, thereby affecting print quality. Finally, two other problems which occur with the use of prior art ink metering systems are "ghosting" and "starvation." Ghosting and starvation are caused by the excess ink on the image or plate cylinder which does not get transferred to the paper in areas where there is no image to be printed. This excess ink, or conversely, the lack of ink resulting from the removal of ink for printing the image, can cause variations in the ink film depth size on the image cylinder in subsequent printings. Such variations in ink film depth size can alter the quality of the image printed.
A second device for distributing ink to an image cylinder is a ductor. A ductor is a roll which is not kept in constant contact with the fountain roll. Instead the ductor oscillates between the fountain roll and the image cylinder. When the ductor contacts the fountain roll, it rotates at the speed of the fountain roll; when the ductor contacts the image cylinder it rotates at the speed of the press. As will be readily understood, the mechanism used to drive the ductor is necessarily complicated and expensive.